Prins-arylthiolation reaction on terpenoids: Diastereoselective synthesis of 4-arylthiooctahydro-2h-chromenes

Article abstract of DOI:10.1002/ejoc.201403080

(-)-Isopulegol undergoes Prins-arylthiolation with different aldehydes and cyclohexanone in the presence of HBF₄·OEt₂ to generate a library of 4-arylthiooctahydro-2H-chromene derivatives in good to excellent yields with moderate to high cis selectivity at ambient temperature in dichloromethane. Readily available HBF₄·OEt₂ as a catalyst outshines CF₃CO₂H, which is generally used in excess amounts in Prins cyclization and domino Prins-arylthiolation reactions.

Full text of DOI:10.1002/ejoc.201403080

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403080
Prins–Arylthiolation Reaction on Terpenoids: Diastereoselective Synthesis of 4-
Arylthiooctahydro-2H-chromenes
Barnali Sarmah,[a] Gakul Baishya,*[a] and Rajani K. Baruah[b]
Keywords: Cyclization / Fused-ring systems / Multicomponent reactions / Nitrogen heterocycles / Terpenoids
(–)-Isopulegol undergoes Prins-arylthiolation with different
aldehydes and cyclohexanone in the presence of HBF4·OEt2
to generate a library of 4-arylthiooctahydro-2H-chromene
derivatives in good to excellent yields with moderate to high
cis selectivity at ambient temperature in dichloromethane.
Readily available HBF4·OEt2 as catalyst outshines
CF3CO2H, which is generally used in excess amounts in
Prins cyclization and domino Prins-arylthiolation reactions.
a
are the use of expensive metal triflates as the catalyst and
excess amounts of the acid promoter.
Introduction
One-pot, three- or multicomponent reactions can be used
in various applications of pharmaceutical chemistry such as
in the synthesis of diverse structural scaffolds and combina-
torial libraries for drug discovery.[1] Similarly, tandem reac-
tions are also very important synthetic methods that ease
the synthesis of different complex organic molecules.[2] The
Prins cyclization reaction for the synthesis of diverse tetra-
hydropyran derivatives is well established.[3] Numerous pub-
lications highlight the utilization of the concept of Prins
cyclization in the synthesis of various biologically and syn-
thetically important molecules such as avermectins, aplysia-
toxin, oscillatoxins, latrunculins, talaromycins, acutiphycins,
apicularens, phorboxazoles, and bryostatins.[4] As is known
from the mechanism of the Prins cyclization reaction, a
number of nucleophiles can quench the carbocation formed
during the course of the reaction to generate different li-
braries of organic molecules.[5] However, there are only two
reports in which inorganic thiocyanate and arenethiols have
been used as nucleophiles to quench the Prins cyclization
reaction to give 4-thiocyanatotetrahydropyran and 4-aryl-
thiotetrahydropyran derivatives, respectively.[6,7] For the
synthesis of 4-thiocyanatotetrahydropyran derivatives, in-
dium(III) trifluoromethanesulfonate [In(OTf)3, 10 mol-%][8]
has been used as a catalyst to perform the Prins cyclization
reaction in refluxing dichloromethane, whereas an excess
amount of trifluoroacetic acid promoted the Prins–thiol-
ation reaction to afford 4-arylthiotetrahydropyran deriva-
tives (Scheme 1). The main drawbacks of these two methods
Scheme 1. Prins–arylthiolation of homoallyl alcohol with arene-
carbaldehydes by using an excess amount of CF3COOH.[7]
Again, 2-mercaptobenzothiazole derivatives[9] are an im-
portant class of compounds that show potent antibacterial
and antifungal activities. Some of them are also known to
exhibit different biological activities, including antitubercu-
lar, -inflammatory, -tumor, -amoebic, -parkinsonian, -thel-
mintic, -hypertsensive, -hyperlipidemic, -ulcer, chemopro-
tective, and selective CCR3 receptor antagonist activity.[9]
Although the concept of Prins cyclization has been exten-
sively utilized for the synthesis of diverse organic structural
frameworks, the introduction of an arylthio functionality
on (–)-isopulegol as a monoterpene to furnish a library of
novel 4-arylthio-octahydro-2H-chromene derivatives is not
known (Scheme 2).
Scheme 2. Prins–arylthiolation of (–)-isopulegol with aldehydes by
using HBF4·OEt2 (present report).
[a] Natural Products Chemistry Division, CSIR – North East
Institute of Science and Technology,
Recently, HBF4·OEt2 has attracted immense attention as
an efficient fluorinating agent and catalyst as well as an
acid promoter to drive a number of organic transforma-
tions.[10,11] This reagent is also known to catalyze the Prins
cyclization and Hosomi–Sakurai–Prins reaction to generate
different libraries of 4-fluorotetrahydropyran[12] and 4-fluo-
ropiperidine derivatives.[13] In continuation of our research
Jorhat 785006, Assam, India
E-mail: b.gakul@gmail.com
http://www.rrljorhat.res.in
[b] Analytical Chemistry Division, CSIR – North East Institute of
Science and Technology,
Jorhat 785006, Assam, India
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403080.
Eur. J. Org. Chem. 2014, 7561–7565
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Sarmah, G. Baishya, R. K. Baruah
SHORT COMMUNICATION
program on the development of novel synthetic methods by p-anisaldehyde (2d, 1.0 mmol), 2-mercaptobenzothiazole
employing the three-component Prins cyclization reaction (3a, 1.2 mmol), and HBF4·OEt2 (20 mol-%) at 0 °C in
to generate different libraries of compounds,[14] herein we CH2Cl2.
wish to report the first example of the HBF4·OEt2-cata-
Once the optimal reaction conditions were known, we
lyzed three-component domino Prins–arylthiolation reac- explored the scope of our protocol for the synthesis of 4-
tion of (–)-isopulegol with different aldehydes and arylthi- arylthio-octahydro-2H-chromene derivatives. First, dif-
ols to produce 4-arylthiooctahydro-2H-chromene deriva- ferent aromatic aldehydes were treated with 1 and 3a to give
tives (Scheme 2).
their corresponding 4-thiabenzothiazolo-octahydro-2H-
chromenes. Aromatic aldehydes bearing both electron-do-
nating and electron-withdrawing substituents underwent
the domino Prins–arylthiolation reaction smoothly to af-
ford their products in very good yields with good diastereo-
selectivities (Table 2, Entries 4–8). Unsubstituted aldehydes
such as benzaldehyde (2a), 2-naphthaldehyde (2b), and ste-
rically hindered 9-anthracenecarbaldehyde (2c) also fur-
nished their corresponding 4-thiabenzothialzolo-octahydro-
2H-chromene derivatives in very good yields with good dia-
stereoselectivities (Table 2, Entries 1–3). Aliphatic aldehydes
such as propanal (2l), phenylpropanal (2m), and isobutyr-
aldehyde (2n) were also treated with 1 and 3a under the
optimal reaction conditions to give their respective prod-
ucts in good yields (Table 2, Entries 12–14). However, the
two diastereomers obtained in each case of the aliphatic
aldehydes were not separable by simple column chromatog-
raphy, and the diastereomeric ratio was determined by
1H NMR spectroscopy.
Results and Discussion
Initially, we attempted the Prins–arylthiolation reaction
of (–)-isopulegol (1) with p-anisaldehyde (2d) in the pres-
ence of 2-mercaptobenzothiazole (3a) as the external nu-
cleophile by using HBF4·OEt2 (10 mol-%) as the catalyst.
Thus, the reaction was performed in CH2Cl2 at room tem-
perature, which gave a diastereomeric mixture of products
4d and 5d in 50% yield (Table 1, Entry 1). The two dia-
stereomers were separated by column chromatography. The
reaction also gave a diastereomeric mixture of Prins-cy-
clized products 6d and 7d in 12% yield along with 4d and
5d. To reduce the yield of products 6d and 7d, we performed
the reaction at a lower temperature for the stipulated time.
As expected, the yield of Prins products 6d and 7d (6%
yield) decreased, which thereby increased the yield of Prins–
arylthiolated products 4d and 5d up to 61% (Table 1, En-
try 2). Upon repeating the reaction by using 20 mol-% of
HBF4·OEt2 at 0 °C, a significant increase in the yield was
observed, and diastereomers 4d and 5d were obtained in
80% yield (Table 1, Entry 3). In subsequent experiments,
the reaction was also performed in different organic sol-
vents, including CHCl3, ClCH2CH2Cl, and hexane, but no
increase in the yield of the Prins–arylthiolated products was
observed (Table 1, Entries 4–6). Thus, the optimal reaction
conditions involve the use of (–)-isopulegol (1, 1.0 mmol),
Table 2. Synthesis of 4-thiabenzothiazolo-octahydro-2H-chromens
by Prins–arylthiolation of (–)-isopulegol with different aldehydes.
Entry
R
Product
Ratio[a] 4/5 Yield [%][b]
Table 1. Optimization of the reaction conditions by using (–)-isopu-
1
2
3
4
5
6
7
8
Ph (2a)
4a + 5a
4b + 5b
4c + 5c
4d + 5d
4e + 5e
4f + 5f
4g + 5g
4h + 5h
4i + 5i
3:1
4:1
3:1
7:1
6:1
5:1
4:1
5:1
78
75
70
80
78
73
65
74
53
58
62
61
69
68
40
legol (1) and p-anisaldehyde (2d).[a]
2-naphthyl (2b)
9-anthracenyl (2c)
4-MeOC6H4 (2d)
4-MeC6H4 (2e)
4-ClC6H4 (2f)
3,4-Cl2C6H3 (2g)
4-BrC6H4 (2h)
2-furfuryl (2i)
2-thiophenyl (2j)
styryl (2k)
9
4:1[c]
1:1[c]
5:1[c]
5:1[c]
9:1[c]
1:1[c]
–
10
11
12
13
14
15
4j + 5j
4k + 5k
4l + 5l
Pr (2l)
C6H5CH2CH2 (2m) 4m + 5m
iPr (2n)
C6H5CH2 (2o)
4n + 5n
4o
Entry
HBF4·OEt2 [mol-%] Conditions
Yield [%][b]
[a] Diastereomeric ratio was obtained from the yield of the isolated
products. [b] Combined yield of the isolated products. [c] Dia-
stereomeric ratio was determined by H NMR spectroscopy.
1
2
3
4
5
6
10
10
20
20
20
20
25 °C/CH2Cl2
0 °C/CH2Cl2
0 °C/CH2Cl2
0 °C/CHCl3
0 °C/ClCH2CH2Cl
0 °C/hexane
50
61
80
75
70
1
Next, we extended our studies to heteroaromatic alde-
hydes. Interestingly, upon treatment of 2-furfuraldehyde (2i)
and 2-thiophenecarbaldehyde (2j) with 1 and 3a under the
same reaction conditions, the corresponding inseparable
diastereomeric mixtures of chromene derivatives (i.e., 4i/5i
n.r.
[a] Reaction performed with 1 (1 mmol), 2d (1.0 mmol), and 3a
(1.2 mmol). [b] Combined yield of isolated 4d and 5d. n.r.: no reac-
tion.
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Diastereoselective Synthesis of 4-Arylthiooctahydro-2H-chromenes
and 4j/5j) were obtained in good yields with moderate dia- Table 3. Synthesis of 4-thio-octahydro-2H-chromenes of 10 and 11.
stereoselectivities (Table 2, Entries 9 and 10). The protocol
was found to be equally effective with the α,β-unsaturated
(E)-cinnamaldehyde (2k), which afforded chromenes 4k/5k
in good yield with good diastereoselectivity (Table 2, En-
try 11). The reaction also proceeded with phenylacetalde-
hyde dimethyl acetal (2o) to afford major diastereomer 4o
in 40% yield; minor diastereomer 5o was not obtained in
its pure form (Table 2, Entry 15).
Entry
R
Ar
Product
Ratio[a]
10/11
Yield
[%][b]
To examine the efficacy of this protocol with ketones, we
also performed the reaction of 1 with cyclohexanone (8)
and 3a under the optimal reaction conditions. Surprisingly,
the reaction furnished major diastereomer 9 in an excellent
diastereomeric ratio of 13:1, as confirmed by 1H NMR
spectroscopy (Scheme 3).
1
2
3
4
4-ClC6H4
(2f)
4-ClC6H4
(2f)
Et (2l)
4-ClC6H4
(3b)
4-MeC6H4
(3c)
4-ClC6H4
(3b)
4-MeC6H4
(3c)
10a + 11a
10b + 11b
10c
13:1
8:1
–
60
66
50
57
Et (2l)
10d
–
[a] Diastereomeric ratio was obtained by analysis of the product by
1H NMR spectroscopy. [b] Yield of isolated product.
Table 4. Comparative studies of different Brønsted and Lewis acid
catalysts for the synthesis of 4d and 5d in CH2Cl2 at 0 °C.
Entry
Catalyst
Equiv.
Time [h]
Yield [%][a]
Scheme 3. Prins–arylthiolation of (–)-isopulegol with cyclohexan-
one.
1
2
3
4
5
6
7
CF3CO2H
CF3CO2H
CF3SO3H
BF3·OEt2
TMSOTf
I2
1.0
10.0
1.0
0.3
0.3
0.1
0.2
12
6
5
3
5
4
4
37
72
74
65
76
56
59
Next, we focused on the introduction of other arenethiol
functionalities in the C-4 position of the octahydro-2H-
chromenes. Thus, different arenethiols such as p-chloro-
thiophenol (3b) and p-methylthiophenol (3c) were used as
external nucleophiles in the reaction of 1 with different aro-
matic and aliphatic aldehydes. They also underwent smooth
Prins–arylthiolation to give their corresponding products as
diastereomeric mixtures in good yields with excellent dia-
stereoselectivities. The reaction of 1 with 2f and 3b afforded
Prins–arylthiolated products 10a/11a in a diastereomeric ra-
tio of 13:1 (Table 3, Entry 1). Similarly, the reaction with 2f
and 3c also produced major diastereomer 10b with excellent
diastereoselectivity (Table 3, Entry 2). The Prins–arylthiol-
ation of 1 and propanal with 3b and 3c in two different
reactions gave major diastereomers 10c and 10d as the sin-
gle products in each case; the other diastereomers were not
isolated even in trace amounts (Table 3, Entries 3 and 4).
However, side products 12 and 13 were isolated in yields of
10 and 12%, respectively, in each of these two reactions
(Table 3, Entries 3 and 4) as the Markovnikov addition
products. From Table 3 it is clear that the protocol is
equally effective with a diverse set of arenethiols bearing
both electron-withdrawing and electron-donating substitu-
ents in the arene ring.
I2
[a] Combined yield of the isolated products.
The mechanism of the reaction can be explained by the
common cis-selective Prins cyclization pathway, for which
initially formed oxocarbenium ion A first generates tetra-
hydropyranyl tertiary carbocation B. Carbocation B is then
trapped by an external arenethiol nucleophile, preferably
from the equatorial side to give C as the major diastereomer
(Scheme 4). The formation of major isomer 4 was also con-
firmed by single-crystal X-ray crystallography[15] of com-
pound 4d (Figure 1). The structure of minor isomer 5 was
determined by a NOESY experiment of 5e, for which a
strong NOE peak between the C-10 methyl protons and the
4-H proton indicated the presence of an equatorial methyl
group at the C-9 carbon atom (Figure 1).
Other strong acids such as CF3CO2H and CF3SO3H
were also employed as catalysts for this important transfor-
mation, but in each case either an excess or an equivalent
amount of the promoter was required for the reaction to
proceed in the forward direction. The efficacy of other
Lewis acids such as BF3·OEt2, TMSOTf, and I2 was also
examined for the synthesis of 4d and 5d. The results are
presented in the Table 4.
Scheme 4. Proposed mechanism for the Prins–arylthiolation of
(–)-isopulegol.
Eur. J. Org. Chem. 2014, 7561–7565
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7563

B. Sarmah, G. Baishya, R. K. Baruah
SHORT COMMUNICATION
Figure 1. Single-crystal X-ray structure of 4d and NOE diagram of 5e.
Supporting Information (see footnote on the first page of this arti-
cle): Spectral data and copies of the 1H NMR and 13C NMR spec-
tra of all final products.
Conclusions
A variety of aromatic aldehydes bearing both electron-
donating and electron-withdrawing substituents underwent
an HBF4·OEt2-catalyzed Prins–arylthiolation reaction with
(–)-isopulegol and different arenethiols to generate a library
of 4-arylthio-octahydro-2H-chromene derivatives at ambi-
ent temperature. This protocol is equally effective with ali-
phatic aldehydes and α,β-unsaturated aldehyde. Cyclohex-
anone also underwent the Prins–arylthiolation with (–)-iso-
pulegol and 2-mercaptobenzothiazole to afford the spiro-
chromene derivative under the reaction conditions. This
protocol offers a novel diastereoselective route for the syn-
thesis of 4-arylthio-octahydro-2H-chromene derivatives by
using a readily available and also slightly less expensive cat-
alyst. The use of a catalytic amount of HBF4·OEt2 sur-
passes the use of trifluoroacetic acid, which is known to be
employed in excess amounts to promote this domino Prins–
arylthiolation reaction.
Acknowledgments
The authors gratefully acknowledge the Council of Scientific and
Industrial Research (CSIR), New Delhi and the Department of
Science and Technology (DST), New Delhi for research grants
CSC-130 (NAPAHA) and SR/FT/CS-126/2010 to this work and
also for a fellowship to B. S. The Director, CSIR-NEIST, Jorhat,
is thanked for providing facilities for this work.
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General Procedure for the Synthesis of 4-Arylthio-octahydro-2H-
chromene Derivatives: HBF4·OEt2 (0.2 mmol, 0.2 equiv.) was added
slowly at 0 °C to a solution of (–)-isopulegol (1.0 mmol, 1.0 equiv.),
aldehyde (1.0 mmol, 1.0 equiv.), and arenethiol (1.2 mmol,
1.2 equiv.) in dry CH2Cl2 (2 mL), and the mixture was slowly
warmed up to 25 °C with continuous stirring for 3–4 h. Upon com-
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NaHCO3 (10 mL) was added, and the mixture was extracted with
ethyl acetate (3ϫ 10 mL). The organic layer was washed with brine
(1ϫ 10 mL) and dried with anhydrous sodium sulfate. The organic
layer was concentrated under reduced pressure, and purified by col-
umn chromatography (silica gel 100–200 mesh; hexane and ethyl
acetate/hexane, 1:99) to afford pure 4-arylthio-octahydro-2H-
chromene derivatives.
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Received: August 14, 2014
Published Online: October 29, 2014
Eur. J. Org. Chem. 2014, 7561–7565
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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